The presence of multipath interference in a broadcast channel, such as a broadcast television channel, has been a persistent obstacle to good reception by a receiver. The broadcast television channel is a relatively severe multipath environment due to a variety of conditions that are encountered in the channel and at the receiver. Channels are characterized by channel impulse responses which may be several hundreds of symbols long, so that strong interfering signals may arrive at the receiver both before and after the largest amplitude signal. In addition, the signal transmitted through the channel is subject to time varying channel conditions due to the movement of the transmitter and/or due to signal reflectors, airplane flutter, and, for indoor reception, people walking around the room. If mobile reception is desired, movement of the receiver must also be considered.
Multipath adversely affects the ability of the receiver to correctly receive the symbols transmitted by the transmitter. Therefore, equalizers, such as decision feedback equalizers, are used in receivers in order to cancel the effects of multipath and thereby improve signal reception.
The decision feedback equalizer (DFE) is an attractive type of equalizer for this application because it theoretically performs better than a linear equalizer. The decision feedback equalizer typically includes a feed forward filter that includes a plurality of feed forward taps, a feedback filter that includes a plurality of feedback taps, some type of decision device such as a slicer or trellis decoder, a summer, and a tap weight calculator that sets the tap weights for the feed forward filter and the feedback filter.
The received signal is provided to the feed forward filter. The outputs of the feed forward filter and the feedback filter are summed by the summer. The output of the summer forms the output of the decision feedback equalizer. The decision device decides a correct value for the output of the summer and feeds this decision to the feedback filter. The tap weight calculator estimates the channel impulse response based on the received signal and the decisions from the decision device. The tap weight calculator then determines the optimal (MMSE—minimum mean squared error) tap weights based on this channel impulse response estimate and the estimated signal-to-noise ratio that characterizes the received signal, and adjusts the feed forward taps of the feed forward filter and the feedback taps of the feedback filter according to these tap weights.
The performance of decision feedback equalizers can be affected by a phenomenon known as error propagation. Error propagation occurs when a decision error, fed into the feedback filter, causes an additional decision error to occur. In cases of severe intersymbol interference (ISI), the reduction in performance can be so large that steps must be taken to mitigate error propagation. The probability that a decision error will propagate a new decision error is a function of the size of the tap weights of the feedback filter. Larger tap weights increase the probability that a new error will occur as a result of a previous error.
One of the known ways to reduce error propagation is to calculate the MMSE tap weights for the decision feedback equalizer subject to a constraint on the tap weights of the feedback filter, thereby limiting the tap weights of the taps of the feedback filter to values that reduce error propagation. However, the design of a practical decision feedback equalizer that constrains its tap weights to values that reduce error propagation is not intuitive.
The present invention relates to practical designs for decision feedback equalizers that constrain their tap weights to values that reduce error propagation.